2,5-Furandicarboxylic acid (“FDCA”) is a dicarboxylic acid for which the commercial interest has grown recently. The diacid or its diester has been found to be particularly interesting for the preparation of poly(alkylene-2,5-furandicarboxylate), in which preparation FDCA is condensed with an alkylene glycol, such as monoethylene glycol (MEG). The polymer obtained from the polymerization of FDCA or its diester and MEG, polyethylene-2,5-furandicarboxylate (PEF), can be used as alternative for polyethylene terephthalate (PET) or, due to its properties, can be used in areas where PET cannot be used. As one of the advantages of using FDCA it is considered that FDCA is obtainable from sustainable sources. In WO 2011/043661 and U.S. Pat. No. 8,519,167 methods are described wherein 5-hydroxymethyl furfural and derivatives thereof are converted into FDCA and esters thereof. As explained in these patent documents, the starting material 5-hydroxymethylfurfural and derivatives thereof can be obtained from carbohydrate-containing sources, such as fructose, glucose, sucrose, starch, cellulose etc.
US 2012/0302768 discloses that the oxidation process of 5-hydroxymethylfurfural and derivatives thereof leads to the formation of a mixture of 2,5-furan-dicarboxylic acid, 2-formyl-furan-5-carboxylic acid and, optionally, some other furan derivatives, such as alkyl esters of 2-formyl-furan-5-carboxylic acid. Although it is stated that purified 2,5-furandicarboxylic acid can be obtained by washing, it was found that also the purified product still contained an amount of 2-formyl-furan-5-carboxylic acid. It is acknowledged in US 2012/0302768 that significant concentrations of mono-functional molecules like 2-formyl-furan-5-carboxylic acid in the 2,5-furan-dicarboxylic acid product are particularly detrimental to polymerization processes as they may act as chain terminators during a polyester condensation reaction.
It has been found that washing does not yield pure product. It is believed that 2-formyl-furan-5-carboxylic acid is included in the crystals of 2,5-furan-dicarboxylic acid whereby the purification by washing becomes unfeasible. Purification of the corresponding ester products was found to be easier.
Many prior art documents wherein the preparation of poly(alkylene-2,5-furan dicarboxylate) is described, mention that the resulting polymers are colored. In order to be able to compete with transparent PET it is desirable that poly(alkylene-2,5-furandicarboxylate) and thus the starting materials in the preparation thereof are as colorless as possible.
The need for a method for the purification of esterification products of FDCA has been acknowledged in WO 2013/191942. In order to meet this need WO 2013/191942 describes a process wherein an FDCA-containing composition is reacted with an alcohol component, and a portion of the alcohol component is separated from the product of the esterification reaction to obtain an ester composition comprising the dialkyl ester of FDCA as solid material. The ester composition is subsequently purified. The purification of the ester composition comprises solids separation, washing, crystallization, solid-liquid separation, re-dissolution and recrystallization. WO 2013/191942 states that in this way an ester composition can be obtained that contains at least 96% wt of solid dialkyl ester of FDCA and less than 3% wt of the monoalkyl ester of FDCA, less than 3% wt of alkyl-5-formyl-2-furancarboxylate, and less than 1% wt of FDCA and that shows a color component b* of at most 5.
Applicants have found that the mere crystallization of esterified FDCA does not result in a satisfactory removal of compounds that are colored or are precursors of color-producing compounds. By repeating the recrystallization steps it may be possible to obtain a certain level of purity. However, such repetitions will inevitably also result in a low eventual yield, since together with impurities also desired dialkyl ester product will remain in the mother liquor from which the recrystallizations are being carried out. Hence, there is a need for a process for the preparation of dialkyl esters of FDCA, wherein the yield of pure dialkyl ester is improved by avoiding the loss due to one or more recrystallization steps.